Syria threatened Monday to unleash its chemical and biological weapons if the country faces a foreign attack, a desperate warning from a regime that has failed to crush a powerful and strengthening rebellion.

The statement - Syria's first-ever acknowledgement that the country possesses weapons of mass destruction - suggests President Bashar Assad will continue the fight to stay in power, regardless of the cost.

"It would be reprehensible if anybody in Syria is contemplating use of such weapons of mass destruction like chemical weapons," U.N. Secretary-General Ban Ki-moon said during a trip to Belgrade, Serbia. "I sincerely hope the international community will keep an eye on this so that there will be no such things happening."

Syria is believed to have nerve agents as well as mustard gas, Scud missiles capable of delivering these lethal chemicals and a variety of advanced conventional arms, including anti-tank rockets and late-model portable anti-aircraft missiles.

During a televised news conference Monday, Foreign Ministry spokesman Jihad Makdissi stressed that the weapons are secure and would only be used in the case of an external attack.

"No chemical or biological weapons will ever be used, and I repeat, will never be used, during the crisis in Syria no matter what the developments inside Syria," he said. "All of these types of weapons are in storage and under security and the direct supervision of the Syrian armed forces and will never be used unless Syria is exposed to external aggression."

The Syrian government later tried to back off from the announcement, sending journalists an amendment to the prepared statement read out by Makdissi. The amendment said "all of these types of weapons - IF ANY - are in storage and under security." It was an attempt to return to Damascus' position of neither confirming nor denying the existence of non-conventional weapons.

In his comments to reporters, Makdissi also repeated the regime's assertion that the country's 17-month-old conflict, which activists say has killed at least 19,000 people, is not the result of a popular uprising, casting it instead as the work of foreign extremists looking to destroy the nation.

Israel and the U.S. are concerned that Syria's stockpile of chemical weapons could fall into the hands of Islamist militants should the regime in Damascus collapse. Israeli Prime Minister Benjamin Netanyahu warned Sunday that his country would "have to act" if necessary to safeguard the arsenal from rogue elements.

In Washington, State Department spokeswoman Victoria Nuland said Monday that "any possible use of these kinds of weapons would be completely unacceptable."

"The Syrian regime has a responsibility to the world, has a responsibility first and foremost to its own citizens to protect and safeguard those weapons," she said, adding that Washington was working with allies to monitor the situation and send the message to both Syria's government and opposition about the importance of protecting non-conventional weapons.

A senior U.S. intelligence official said Friday the Syrians have moved chemical weapons material from the country's north, where the fighting was fiercest, apparently to both secure and consolidate it, which U.S. officials considered a responsible step.

But there has also been a disturbing rise in activity at the installations, so the U.S. intelligence community is intensifying its monitoring efforts to track the weapons and try to figure out whether the Syrians are trying to use them, the official said on condition of anonymity to discuss the still-evolving investigation.

Concerns over Syria's long-suspected chemical weapons stockpiles have skyrocketed in recent days as the rebels gain serious momentum in their fight to oust the Assad regime.

Since last week, the anti-Assad fighters have claimed a stunning bomb attack that killed four high-level security officials in Damascus, captured several border crossings and launched sustained offensives in Damascus and Aleppo, the two largest cities and both regime strongholds.

Makdissi tried to assure Syrians that the situation was under control, despite reports of clashes throughout the country.

"Yes, there were clashes on certain streets in certain neighborhoods, but the security situation is now much better. Everyone is feeling reassured," he said. "We are not happy about this, but this is an emergency situation and it will not last more than a day or two and the situation will return to normal."

Security forces appeared to show more government control in videos posted online by activists Monday. Some of the clips show Syrian militia sweeping through Damascus neighborhoods once held by rebels, kicking down doors and searching houses in mop up operations against the fighters that had managed to hold parts of the capital for much of last week.

It was a different story in Aleppo, however, where the Britain-based Syria Observatory reported fierce fighting in a string of neighborhoods, including Sakhour and Hanano, in the northeast of Syria's largest city.

Several videos posted by activists showed rebels battling regime tanks in Sakhour's narrow streets. In one clip, a tank on fire rumbles along a road after being hit by rebels as a man jumps out of the flaming turret. Other videos showed cheering rebels celebrating around destroyed tanks, even driving around one they had captured.

The rebel advance has been a swift turnaround in the momentum of the uprising, which began in March 2011. Still, the opposition remains hobbled by divisions within their ranks and the fact that they are outgunned by the well-armed regime. The violence, meanwhile, has become far more unstable than many had ever imagined, with al-Qaida and other extremists exploiting the chaos.

Still, the opposition fighters have kept up their battle for 17 months, chipping away at government power and penetrating the aura of invincibility that the Assad family dynasty has built up over four decades in power.

Gulf nations such as Saudi Arabia and Qatar have pledged funds to aid Syria's rebels, but there is no clear trail showing how much is reaching the fighters.

U.S. officials are debating whether to step up aid to the rebels, including sending in heavy weaponry, but officials are worried the aid may end up in the hands of Islamic militants who have infiltrated the rebel Free Syrian Army, the American official said.

Former CIA officer Reuel Marc Gerecht, who is now a scholar at the Foundation for the Defense of Democracies, said Friday that the agency has only a handful of operatives working on the Turkish side of the Syrian border, helping allies who want to give the rebels aid identify which groups are legitimate.

The agency has distributed encrypted radios to the rebels to help them coordinate their attacks. Gerecht has called for the White House to initiate a covert CIA operation inside Syria, to help arm the rebels with weaponry able to take down the helicopter gunships menacing Syrian towns.

Even as the government appeared to be reasserting control in the capital after the weeklong rebel assault, the Arab League offered Assad and his family a "safe exit" if he steps down.

Assad, 46, is married with three young children under the age of 13.

"This request comes from all the ... Arab states: Step aside," said Qatari Prime Minister Hamid bin Jassim Al Thani at an Arab League foreign ministers meeting in Doha, Qatar, that concluded at dawn Monday. He urged Syria to form a temporary transitional government to plan for a possible post-Assad era. Makdissi dismissed the offer as "flagrant interventionism."

The Arab League has already suspended Syria's membership and it is doubtful that Assad will pay much attention to their calls.